Digital telephone systems have been developed to increase the versatility of a centralized system in an office or building while providing the proper interface to the central telephone office network. These digital telephone systems utilize an analog transmission line and a digital transmission line for connection to a digital telephone set or other digital telephone device. Since the digital telephone system provides a separate interface to the central office telephone network, the termination to the digital telephone set on the digital port thereof does not have to be the conventional -48 volts. In so doing, a particular manufacturer can customize his terminations and voltage levels to suit his manufacturing purposes.
To control processing of telephone signals in the digital telephone system, the digital transmission paths are utilized between the digital telephone system and the digital peripheral devices connected thereto. The digital transmission path can be either a four wire path for simplex transmission or a two wire path for a multiplex transmission. The simplex system provides two wires for receiving digital data and two wires for transmitting digital data from the digital peripheral device back to the digital telephone system.
One disadvantage to digital telephone systems is that they do not provide for the connection of conventional analog telephone devices on the digital port. The voltage levels required for a conventional analog telephone device and the impedance thereof does not facilitate direct connection to the digital port of a digital telephone system. In addition, an analog communication path must also be present in order for analog transmission to be placed on the digital port of the digital telephone system. Therefore, there exists a need for an interface circuit that allows the interface of a conventional analog telephone device with a digital telephone system in conjunction with one of the digital peripheral devices connected to the digital telephone system.